Sustained nuclear fusion in a controlled environment would yield an apparantly inexhaustible supply of energy. Inertial confinement fusion (ICF) requires targets which can maintain the deuterium and tritium (DT) fuel as a uniform liquid layer, which may typically be up to several hundred microns in thickness. The high gain, single shell, direct drive ICF targets currently under consideration require, under certain conditions, that the fuel be levitated. Maintaining the thick liquid DT layer is quite difficult as the liquid fuel slumps under the influence of gravitational forces.
A suggested approach for maintenance of the thick liquid DT layer is the use of a small pore size, open cell, rigid foam structure which serves as a sponge to define the layer contours and to stabilize the liquid fuel against gravitational slumping. Darling, D. H., U.S. Pat. No. 4,693,938. Both inorganic (e.g. silica, alumina, borosilicate) and organic (e.g. polystyrene) materials have been suggested as appropriate compositions for the foam matrix of ICF targets.
When the target capsule, containing the deuterium-tritium (DT) fuel mixture, is irradiated by laser beams at a sufficiently high energy level, the outer layers of the ablator blow off, which drives the hollow capsule inward, causing an implosion which heats the fuel. At appropriate conditions, the fusion of the deuterium and tritium fuels is initiated and propagated, yielding large amounts of energy in the form of highly energetic neutrons.
One target design requires a hollow sphere or shell, in which the wall is a low density, porous material which can serve as a reservoir for the liquid fuel. Darling, D. H., U.S. Pat. No. 4,693,938. Further, the wall material of the hollow sphere should be sufficiently strong to support the fuel-loaded sphere during acceleration and injection of the hollow sphere into the ICF reactor.
Hollow spheres of porous inorganic materials appear to be especially suitable for formation of ICF target shells. The use of inorganic materials, such as silica, enables formation of porous, yet strong microspheres of the appropriate physical dimensions.
Several approaches have been taken to form aerogels into desired shapes. Prior work to make formed elements has produced droplets of spheroidal shape, but not hollow spheres of controlled size.
U.S. Pat. No. 2,463,467 by M. M. Marisic disclosed a method for producing strong inorganic oxide aerogel pellets by spray drying through an orifice to form aerogel globules which are of large size, one-half inch in diameter. The colloidal solution was ejected into a water-immisible fluid or air.
U.S. Pat. No. 3,245,918 by A. J. Burzynski disclosed production of solid silica beads from alkyl orthosilicates. The bead size ranged from 1 micron to 1.5 mm in diameter.
U.S Pat. No. 3,161,468 by R. J. Walsh disclosed the formation of hollow spherical shells of small size by a flame combustion process. The silica sol was atomized into a flame of 600.degree. to 1800.degree. C. to produce hollow spherical shells of silica which were 0.2 to 20 microns in diameter, but there was no specific regulation of the diameter. The shell composition was not porous silica aerogel.
U.S. Pat. No. 2,834,739 by Reeves et al. disclosed production of inorganic oxide gels from furnace slags. Microspheroidal gel particles of low density were made by spraying silica-alumina hydrosol droplets through an atmosphere of air and ammonia in order to increase the pH to an optimum for formation of a gel. The method described formation of synthetic oxide gels with densities of 0.9 to 1.0 whereas aerogels have densities in the range of about 0.020 to 0.3 g/cm.sup.3.
Hollow glass microspheres have been made by the vertical-drop furnace liquid-drop process in which solvent evaporates as the liquid is dropped through various temperature gradients, as was described by A. Rosencwaig et al. in U.S. Pat. No. 4,257,799. Similar production techniques have been used on hollow polystyrene spheres Both glass and polystyrene microspheres can fall to the bottom of the drop-tower and are sturdy enough to be recovered without damage.
The methods previously described are inadequate for making hollow aerogel microspheres which have the desirable properties of strength, but porous aerogel walls.